Pulse code modulation system



Nov. 1, 1955 Filed April 29, 1952 J. P. JONES, JR

PULSE CODE MODULATION SYSTEM Nov. l, 1955 J. P. JONES, JR

PULSE CODE MODULATION SYSTEM 3 Sheets-Sheet 2 Filed April 29, 1952 Nov.l, 1955 J. P. JONES, JR 2,722,560

PULSE CODE MODULATION SYSTEM Filed April 29, 1952 3 Sheets-Sheet 3ATTORNEY United States Patent() PULSE CODE MODULATION SYSTEM John P.Jones, Jr., Pottstown, Pa., assignor to the United States of America asrepresented-byv the Secretary of the Army Application April 29, 1952,Serial No. y285,036

13 Claims. `(Cl.` 332-11) The present invention relates to electricalsystems and more particularly to coding systems for transforming asignal into consecutive groups of code pulses indicative of consecutivevalues of the amplitude of the signal.

The invention will be specifically described in connection with a systemin which the pulses of the code groups are arranged so as to correspondto Adigits of the binary system. However, it should be well understoodthat the invention is also applicable'to systems in` which the pulses ofthe code groups are arranged in any other desiredl manner whereby therepetition rate, the time-space position and/or the -duration of thepulses of consecutive code groups serve to indicate the consecutiveamplitude values of the signal.

It has been proposed to produce coded pulse groups indicative of theamplitude values of a known signal by means of a so-calledl coding tube.In its most usual form, such a coding tube'consists essentially of acathoderay tube having a perforated target plate and a collectorelectrode for the electrons passing through the perforations. The targetplate is perforated in a rectilinear pattern soar-rangedl that as thecathode-ray beam scans the target plate in one direction it traverses aseries of apertures corresponding to a particular code group. Bysuccessively displacing the cathode-ray beam normal to the scanningdirection to other positions on the target plate as determined by theamplitude of the signal to be coded, additional code groups each`representative ofa specific amplitude value of the signal may beproduced.

In order to allow suicient time for the coding process, the signal to becoded is initially quantized by a suitable input circuit so thatVitsfamplitude variationsA are converted from a continuously varyingtunction into a steplike function. Each successive step of this functionmay represent alinear variation of the amplitude of the signal to becoded. or, if desired, thesuccessive steps may represent non-linear i.e., logarithmic'variations of the signal amplitude. The number ofdiscretelevels to which the signal may be quantized is established bythe capabilities of the coding system. For example, in a coding systemin which the pulses are arranged so as to correspond to the digits of abinary' number system and in which each code group may contain akmaximum ot four pulses, 'the signal to be coded may be quantized to 1'6discrete levels. Similarly, in a binary coding system in which each codegroup may contain. a maximum of seven pulses, 'the signal to be codedmay be quantized to 128 discrete levels.

Pulse coding systems of the foregoing described type have been describedby L. A. Meacham and E. Peterson in the Bell System Technical Journal,Januaryx194'8 at pages l to 43 thereofL andthe coding tube has-beendescribed by R.y W. Sears in the accompanying article of thispublication at pages i4-57 thereof.l

Coding systems of` the foregoingL typey arecharacterized by severalimportant disadvantages., More particularly, the target plate of thecoding tube musty be made with great precision if accurately formedpulsegroups are to be generated thereby. Furthermore, to-prevent a false 'iceresponse, i. e., to insure that the proper series of apertures isselected by the scanning beam, the quantized signal applied to the tubemust have accurately established step values within smalltolerances andthe sensitivity of the deection system of the tube must be similarlyclosely controlled. In addition,'suitable scanning signals and beamaccelerating potentials must be supplied to the coding tube. Theserequirements necessitate the use of complex and space consumingauxiliary equipment which limits the coding system `to relatively fixedlocations and makes it unfeasable for mobile and particularly air-borneuse. Another disadvantage of such prior coding systems is that thecoding tube is relatively fragile and susceptible to shock therebyfurtherrestricting such prior systems from use with mobile and air-borneequipment.

It is an object ofthe invention to provide a pulse coding system ofgreat precision and reliability.

A further object of the invention is to provide a pulse coding systemwhich is compact, rugged and of low cost.

Further objects of the invention will appear as the specicationprogresses.

In accordance with the invention the foregoing objects are achieved bymeans of a novel pulse coding system comprising a pulse code generatingcomponent, by means of which the various code groups to be transmittedare simultaneousl-y generated'in a continuous manner, and an amplituderesponsive selectorv component by means of which the so generated pulsecode groups are selctively supplied to the transmission path'inconsecutive order as established by the amplitude of the intelligencesignal applied tothe amplitudel selector.

ln a preferred form of a system of the invention, wherein the codegroups are constitutedV by pulses corresponding to digits ofthe binarynumber system, the pulse code generating component comprisesv individualcircuit portions, each adapted to produce one of the pulses constitutingthe code groups. The pulses'from each circuit portion are generated at arate equal to a predetermined pulse group repetition rate and thepulsesy from the-individual circuit portions are arranged withpredetermined time-spaced positions relative to each'other. By means ofappropriate connections to the respective circuit portions, there aremade available at the output of the generator a plurality of codegroups, each ofwhich may be used as a measure of a particular amplitudeof the signal to be coded. In its preferred form, the system of theinvention further comprises a code group selector embodying a pluralityof gates which are arranged in cascade and the individual gates of whichmay be consecutively opened to the exclusion of the remaining gates asdetermined by the amplitude value of an applied cont-rol signal; Bycoupling each of the code groups to a different one of the gates, thecode groups may be coupled to the transmission system in consecutiveorder as determined by the consecutive amplitude values of the controlsignal. l

The control signal for actuating the code group selector is derived froma quantizer by means of which the variations of thesignal to be codedare converted into a steplike function at a rate equal to the pulsegroup repetition rate.

The invention will be described in greater detail with referenceto theappended drawings forming part of the specification and in which:

Figure l is ablock diagram of a coding system in accordance withthe-invention;

Figure 2 is a schematic diagram of one form of a pulse code groupgenerator suitable for the' system of the invention;

Figure 3' is a schematic diagram of one form ofa code group selectorsuitable for the system of the invention;

Figure 4 is a schematic diagram of one form of a quantizer suitable forthe system of the invention; and

Figure 5 is a detailed block diagram of one form of a signal source forsynchronizing the operations of the code group generator and thequantizer of Figures 2 and 4 respectively.

Referring to Figure 1, the pulse coding system there shown comprises acode group generator serving as a source of code groups which arecontinuously generated at a predetermined rate as established by therequirements of the system, and which are continuously available forselection. Each of the code groups may consist of one or more pulsesarranged so as to correspond to the digits of the binary system so thateach of the groups corresponds to a different amplitude value of thesignal to be coded. in the case of a four digit binary coding system,the code groups comprise a maximum of four pulses so that 16 amplitudevalues of the signal to be coded may be represented. Similarly, in thecase of a seven digit coding system, the code groups comprise a maximumof seven pulses permitting 128 different amplitude values of the signalto be coded to be represented. And in the general case, in an n digitcoding system, the code groups comprise a maximum of rz pulsespermitting different amplitude values of signal to be coded to berepresented, where p equals the number of pulses combined and may be anyinteger from zero to n. In the preferred arrangement of the inventionand in the interests of simplicity and economy, the various code groupsare constructed from a common source supplying the pulses atpredetermined phase intervals and at the pulse group repetition rate aslater to be specificaly described.

The pulse groups which are continuously available rom the generator 1i),as above described, are supplied to a code group selector 12, thefunction of which is to consecutively select the available code groupsone at a time in time sequence and in an order as determined by theconsecutive amplitude values of a control signal applied to the selector12. Selector i2 may consist essentially of a plurality of gates havingindividual input circuits each energized by a different one of the codegroups and a common output circuit from which the coded signal to betransmitted is derived. The gates are arranged in cascade and mayfurther comprise an additional input system to which a control signal isapplied for selectively opening the gates, the cascade arrangement beingso devised that, for each amplitude value of the control signal, only acorresponding one of the gates is actuated and only the correspondingcode group is transmitted from the generator it) to the output of theselector.

The signal to be coded serves as a control signal for the selector 12and is supplied thereto preferably in the form of a step-like wave. Forconverting the signal to be coded from a continuously varying functioninto a step-like function there is provided a quantizer 11i whichsamples the signal applied thereto at recurrent predetermined intervals,the amplitudes of the samples so taken being desirably maintained at asubstanially constant value between sampling intervals in order toafford suiiicient time for the subsequent coding operation. The rate atwhich the signal to be coded is sampled is determined by the highestfrequency component thereof to be transmitted and, as a practicalmatter, the sampling rate should not be les than twice this highestfrequency component of the signal, as is well known to those skilled inthe art.

The generator 10 and the quantizer 14 are operated in synchronism andfor controlling these components there is provided a synchronizingsignal generator 16.

One specic form of a code group generator suitable for the system of theinvention is shown in Figure 2. in the following description a four digtbinary coding system operating at a code group repetition rate of 6kc./sec. will be assumed. Accordingly, the generator shown in Cil Figure2 is constructed so as to generate four time-spaced pulses, the grouprepetition rate of which is equal to the said specied rate. Forproducing four digit code pulses, the system shown in Figure 2 comprisesrst and second square wave sources 20 and 22 respectively, each of whichcomprises an Eccles-Jordan type trigger circuit of a form well known tothose skilled in the art. In the specific form shown, the oscillator 20comprises two electron discharge tubes 24 and 26 having their anode andgrid circuits interconnected by means of resistancecapacitance networks28 and 30. The anodes of the tubes 24 and 26 are energized by a suitablesource of positive potential (not shown) through load resistors 32 and34 respectively, whereas the grids of the tubes are interconnected bygrid leak resistors 36 and 38, to the junction of which a synchronizingsignal for controlling the frequency and phase of the source is applied.

As later will be more fully discussed, the synchronizing signal appliedto the source 2t) has a frequency equal to twice the repetition rate ofthe pulse code groups. in the specific example herein illustrated thefrequency of the synchronizing signal is equal to l2 kc./sec.

At the anode of tube 24 a rst square wave at a frequency of 6 ltd/sec.is obtained. This wave, when applied to a differentiating networkconsisting of a capacitor 40 and a resistor 42, produces short durationvoltage pulses coincident with the leading and lagging edges of thesquare wave. The negative going differentiated pulses are selected by aclipper tube system 44a also serving as an isolation amplier, andappearing in the output circuit of the clipper system as a positivegoing pulse indicated by the reference numeral 1.

The design of the isolation amplifier-clipper 44a conforms to standardpractice and, in the arrangement shown, consists of a triode electrondischarge tube 46, the anode of which is energized from a source ofpositive potential (not shown) through a resistor 48, and the grid ofwhich is normally maintained at a positive bias value through a gridleak resistor 5? connected to the anode.

At the anode of the tube 26 a second square wave is obtained which is180 out of phase from the square wave produced at the anode of tube 24.This second square wave is similarly applied to a differentiatingnetwork 40a-42a and thereafter supplied to an isolationamplifier-clipper 44h to produce an output positive going pulseindicated by the numeral 3.

The source 22 is identical to source 20 and the anodes thereof supplytwo additional square waves phased 180 relative to each other. Thesesquare waves, in turn. are differentiated in the manner above describedand, by means of isolation amplifier-clippers 44C and 44d, produce twopositive going pulses indicated by the numerals 2 and 4 respectively.

The respective time-phase positions of the pulses 1, 3, 2 and 4appearing at the outputs of the clippers 44a, 44h, 44C and 44d have beenindicated by the diagrams arranged adjacent to the clippers. It will benoted that the pulses occur at uniform intervals throughout the durationof the pulse group, i. e., consecutive pulses are spaced apart.

To produce this quadrature relationship between the pulses 1 and 3 onthe one hand and the pulses 2 and on the other hand, the source 22 ismade to operate in quadrature to the source 20. This is readilyaccomplished by adjusting the synchronizing signals supplied to thesources 20 and 22 so that these signals are phase displaced relative toeach other.

By means of suitable isolation resistors coupled to the outputs of theamplifier-clippers the four spaced pulses 1, 2, 3 and 4 are arranged invarious binary digit combinations to produce a plurality of pulse codegroups, each code group corresponding to a different amplitude value ofthe signal to be coded. More particularly, the code group 1, 0, 0, 0 maybe derived from the g Qr generatorby means of anxi'solation resistor2-1-\`coupled' erator -atf a point ofthevgenerator producing-the code t.i tothe output 'off'amplier-cl-pper '4`4a. {The-f code groupvy`group'correspondingito the` second ofthe-amplitude values A0,13*0, 0'maybe-'clrived-ffrornl the-"generator by meansy to be indicated;i.e.-,to=-thefree endofffresistor 23 (see of an-isolation `Jresistor-123'ifouplede tof' theoutput f Figure 2) 'at which point'lthecode group*0, 1,'0, 0 is i arnplif'levclipper 44c;`the'c'o'de group il, 13H0,Ofvma'y be--5 available derived at"the'junetionfofisolation resstors--25and Thef third and succeeding vgat'es,'with fthe exception of 27 whicharecoupled tothe outputs ofiamplifierclippers-K f theflast' gate' of'the'lcascade' system, are constituted in ^44a`and 4de-res'pectii/ely.`jrInsimilarmanner,fcode groupsl the mannerabove describedin connection'with the second iwithjthe'digit pulses arranged in thesva'rioustothercorngate and are coupled 'in cascade in the 'same manner as -binationsare'producedbyfcombining the outputs of lone'l 10"describedin-connectionwith' theuii'rst and second gates. \or more oftheamplier-'clippers For' the sake of com-Y `Similarly,the'secondffcontrol'grids'ofleach of-these gates `pleteness, thel codegroup d, 1, 11,f1fr which represents'f areeachvconnected to a diiierentone of the points of the "M thetrnaximum"eapability ofl-ftliefpartculargenerator de-1 lflgenerator at Which-the various other codev groups arescribed,I has beenillustrated I="'I7his--pulse groupgas willf-`available. In the :case ofthe third gate, the-'second con- 1 be noted,may 4'be*pr'oducedbyi combininglthelfoutputs 152 trollg-ridthereofisconnected tothe junction ofthe resistors of allofthe=arnpliersclippers"through isolation resistors* -v f25 and -27('see' Eigure) 'at Which1point=-the code group es 29; 31, 335iand 35. f1,l,0,0 is available.

"Thetour-digit'system' hereinedescribd; makes avail- The lastofthelseriesof gatesconstituting the selector able '"1'5 different pulsegioups-byf means of: which 16A l comprisesv an -electronfdischarge tube1121 having a cathdifrerent arnplitude values of the signal to becoded'may20 "o'dellf1l14' vconnected to: a point 'at ground potential,an be represented, anode 116 connected in common with the anode 54 ofpaticulrly'eiective ycode grupf sele`ctor for the the tubeVSO to theload resistor 62,\andl rst,fsecond and In 4the f0rm- Shown, thecselecmrimpriSeSa-a: plurality' 'l' grid ll'isenergiz'ed by the control signalAfor the selector p .tof gates Whichare connectednin.@Cascade:and each Of25 f'through a grid resistor 124 in al manner later-to be more which,when?suitably#actuatedglisadaptedfto suppl-'y fullydescribedywhereaszgrid 120fis connected to the a; different one of thecodegroups-toafcomrnon output jUIi'fiOll 126ff0f-`fa-TvesistiVe neW0rksh0WI1 in Pari); the

us5/Stem', For'the binryA coding System hereinspecjfeallyi latternetworkbeing similar to` the resistor network 68,

- described, the "selector 'eornprisesa f1l5 gates, leach for a 1 70anfi- 72=alid the fesisOI-` ileWOIk'98, 100 end 102 PfeVi different oneof theicode: groups.'produeedfbytthe'gen- 30 ously describ'edrand beingsimilarly connected across a erator of FigureZ.Forlpurposesfof'simplification only* soufeeof" POelifisll'OlOvvshewlfthe Opposite Poles 0f the first three gatesandy-thetalastvgazOf1the,a;cascad i. il which amat- 150 and #|4150 voltsWith respect to ground System are ShoWn c potentialylhegrid122,1servinglas asecond control grid,

z fphetrst offthegafes offthg-Selector shownt-x3-mimises :visprovidedvvitha D.- C; return to thefcathode by means 1 .anflectronfdischrge-ffuben50f.havingggaf'cafode 52, an .35."ofgridresistorf128andis-coupled through acapacitor 130 node '54 and nrstgseeondandcthi'rdf'grids 56,158 and 'f fto'thefcodesroup'fgenerawratapointthereof producing 60 respectively. `4IA'Cathode 52fis1fconn`ected ton-a point fithe Code glOuPCDfTesPOiidiigithe largest amplitude to a source of positive potential +150 through-iaload resistors 29, 31,-5337'ia1d` 35,'a Which Peint the Code resistof-62. The'ffgridfsyservingfas afnrst controlvgridte groupls-Llfllisl'available for theltube :50,'is 'energized by -thefcontrolfsignal applied ffllll'e'fsysiem'sO fal desCfibedS'OPelaiesO selectivelyOPen tofzthe .Selector through' a-grid reg'fstr 64; in agmanner I, ea'choffthegates. tortheiexclusion' of the others so that later tombe,dscl-ibedt- Whereassfhew-fgrid 58,1` Serving as":Y,the'codegroupstapplied toftheisecond controlelectrodes .asclteenvgrid; s Connectewma, sommf; positive P025 may be selectivelymade to appear acrossthe common load tentia'l +150 throughf'ascreenlvoltfagefdroppingresistor f fesistOr I6.2:`as.".deterrnined'byhef'ampliude Value 0f the ,f; *66 Thegfgrd- ,60,ServingsaseafsecondgContr-01grid, ist' l control slgnal applied" to.the1'rst"'contro1electrodes of 2 oupled` to afjuetion 74 -Of@awesistivenhetwgrk Contr thegates. f-a'For this-purpose the rst controlelectrodes-prising\resis'to's68; 70,fand-72 eomectedffinSeries relai *0f eelell0f-"i5l1eiubes'5030,`eiele1^e-`sUPPlledWith PIO- .onshipacrossffatsourceiofnpbtentialmnm shown), thef gressively negativeibiasuvoltagessoithatftube` 50conducts opposite Vp'olesv- Ofk which'arei'gt 11250 ind ati-150.51013" 0 lonly whenffth'efgndVSefthereofiszenergizedibyta' positive With=respectfttg.fgmundwpotental,sTheyseG-ond-Jcontrl gomgpotentlal:exceedlngalglvenvalue;tubei80;conducts sgridlm-is, furtherCoupledgbhroughagcapciwrq to th -fonlywhen thegridfizthereof iszenergized byza positivecode group generator at a point of the generator prd.r`gonlg'fpotentral-exceedingithe irstnrnentioned .potential by ducing-the'icode grouprfcorresponding totheiirst of the ""a'givenlemeulli'iiube 81 "GOiISIUCtS' only `when the grid 87 .amplitudevalues to'ibevindicated,ffi. fe.; twthesfree, end "e'thefeoflserle'relzed' by aposlflve"gomsfpotentiaexceeding offesi/Stor ,21;(tseepigure 2), :at Whicwpoinume code: UIvthe conduction offpotential'offtube 80"by"a glvenV amount, gmupz 1, 00s() isayailable`.1f rand fnadlyftu'be.1112-conductsf'only when"the"grid 118 whThefsecond gate ofythe@electorfcompriss am,felectro'A urthereof 1senergrzedhby-'a posltlve gomgpotential exceed- .dischargettubensoryihaving- ,a cathodeszpan, node 821my mingvftheiconductionpotential-ofthe preceedngtube in f l -and lrst,' secondf n'd 'thirdgridfelectedes'@86,y 88, and 60 the ca`scadesysiemfby another gwenValue- 1 A90 respectively#Cathode 83fisf'connected tofa Point "111.wfourvdlglibmary @dwg-System ofithe'vtype herem atsgmundmotenl,-Whereas-th-e fnfode842.55.onnectdftf.-Speclcallyt-vdescrlbed, vvlnch-is-eapableof-dening fifteen in common-with `the 'node54fzofzcthe1tube 50 to th@ ff-d1screte amplitudevaluesinladditiontothevalue'zero, the ...Joa-d fresistorT62,g'fTheigrida86,3servingsas afirst comm] biaspotentials appliedtowthe rst='control grids of thezfgrid, `is energizedaby the@ central. Signal ,applied to., met Afffsuccessive gates'farek adjusted:to-:fincreasingiincrementalselector'ztinf'ia manner `later toebe :morezpfully described'l il"Valueslasfesablishedbyflhe'maXimum'Peakvelue 0f the z throughafgridrresistor`92',:whereas-grid1z8-8xfis--connected "f Control sigIalapplied 10" heseleeioi and' by ille desired i toftheujunction 94 of: theresistors H70 and-72.; .The grid v a' coding frelationship;f-Forexample:zassumingzuthat the 90, serving asansecondcQntrolizgridgziswcoupled to awe signal-:to becodedxhasvamaximurn'valu'e of'l'lSfvolts and 5, junction 96 of-aresistive-networkcomprising resistors f:7-0 a lineari-*coding relationship isttoxbemaintained, then the s, `.98,f100t:and 1.02 connectedfinseriesmelationship across@ rstfcontrolrgrid ofb'tube 50 -mayebeoperatedlata negaa source of .potentialfY (noti shown),f`athe'ifoppositepoles en' tive potentialequalitohesum of the cut-off bias value of ofwhich are-'at .-:150f and' l1-1250wvo1tsfwithwrespect to ",zrthe tubeAplus'one volt, ith'e'rst controligridof tube 80 is ground potential."The=secondfcontro1fgrid90is furtherzfs =operatedatza*negativefpotentalequal tothesum of thel1vcoupled:througtrafcapacitor`104 towtherco'degroup gen-5575cutfoillbiasfva'lue plustwo fvolts,thelfcontrolsgrid of the third tubeof the cascade series is operated at a negative potential equal to thesum of the cut-off bias plus three volts, etc., until finally thecontrol grid of the last tube is Operated at a negative potential equalto the sum of the cutoff bias plus fifteen volts. When other codingrelationships are used, i. e., a logarithmic relationship, theincrements between the biasing potentials of the first control grids ofthe succeeding tubes of the cascade system differ by amounts which arelogarithmically related.

The bias potentials for the first control grids of the successive tubesare established by individual potentiometcrs 140 which are connected inshunt relationship and form part of a resistor network furtherconstituted by a series resistor 146 and a second series resistor 148.The series network so formed is connected across a source of potential(not shown), the opposite poles of which are at 150 and +150 volts withrespect to ground potential. By adjusting the tapping position of thepotentiometers, the bias potentials of the succeeding tubes of theselector may be adjusted to the desired values as above discussed.

The control signal for the selector is applied to the first controlgrids of the tubes by means of an isolation amplifier 152, the loadimpedance of which is constituted by the above mentioned resistor 146.It will be apparent I to those skilled in the art that by applying asignal to the input of amplifier 152 a corresponding change is effectedin the output circuit of the amplifier so that, for increasinglynegative values of the signal applied to the amplifier, the voltageacross resistor 146, and hence the voltages applied to the first controlgrids of the tubes, become increasingly positive in value. Thus, for asignal of given amplitude level applied to the amplifier 152, one ormore of the first control grids of the consecutive tubes 50, 80, etc.,will attain potential values such as normally to bring about conductionthrough the tubes, the number of consecutive tubes following the tubewhich are placed in this condition being determined by the amplitude ofthe applied input signal.

The selector of Figure 3 is arranged so that only one of the gates isopen at a time as determined by the amplitude of the applied signal, i.e., only the last of those tubes of the series in which the firstcontrol grids are raised to a potential greater than the cut-off valueof the tubes. More particularly, and assuming for the moment that asignal is applied to amplifier 152 of such magnitude that the controlgrids 56 and 86 of the tubes 50 and 80, respectively, are both raised topotentials greater than the cut-ofi values of these tubes as initiallyestablished by the positions of the movable arms of the potentiometers140 and that the signal amplitude is not sufficient to cause conductionin any of the succeeding tubes of the cascade system, under theseconditions tubes S0 and 80 would normally conduct and open the pathsbetween the second control grids 60 and 90 respectively to the commonoutput terminal. However, in order for the selector to perform itsrequired function in accordance with the invention, it is necessary thatthe path through tube 50 be maintained in a closed condition and thatonly the path through tube S0 be opened.

In the system shown, this selective gating is brought about in thefollowing manner: By reason of the positive going signal applied tocontrol grid 86 of tube 80 there is produced a current fiow to thesecond grid 88 of this tube since this grid is operated at a positivepotential. The resultant voltage drop through the resistor 72 causes adrop in the potential of junction 94 which brings about a drop in thepotential of junction 74. In practice the resistors 68, and 72 areadjusted to values at which the voltage at junction 74 is normallyinsufficient to produce cut-off in tube 50, and becomes greater than thecut-off value when the potential of junction 74 is depressed by thecurrent flow to the grid 88 of tube 80.

Similarly, when the amplitude of the signal applied to input amplifier152 has a value such that the first control grids of the tubes 50, and81 are made sufficiently positive to place these tubes in conductingcondition, only the path between the second control grid 91 and theanode is opened. In this instance, the current flow through the tube 81is accomplished by conduction to the grid 89 thereof which is operatedat positive potential through the resistor 102. The accompanying voltagedrop in resistor 102 disturbs the initial voltage distribution atjunctions 95 and 96 causing the potential of junction 96 to assume arelatively large negative value and thereby close the signal pathbetween the control grid and the anode S4. It will also be noted that,notwithstanding the fact that a conductive path to anode 84 is preventedby the negative potential so applied to grid 90, there occursnevertheless a current flow between cathode 82 and the positive grid 88of the tube 80 because of the positive going potential applied to grid86 by the input signal. This current flow to the grid 88 lowers thepotential of junction 94 and causes junction 74 to attain a potentialsufficiently negative to cut ofi the flow of current to thc anode 54 oftube 50.

The remaining gates in the cascade system operate in the manner abovedescribed so that only one gate is opcned at a time, the preceding gatesbiased at a first control grid cut-off potential lower than that of theopened gate being maintained in closed condition by reason of theincreased negative potentials applied to their ysecond control grids bysucceeding gates, and the gates following the conducting gate in theseries being maintained closed by reason of the fact that the appliedsignal amplitude is insufficient to overcome the cut-ofi bias applied tothe first control grids thereof.

The last gate of the series can open only when the amplitude of thecontrol signal attains its maximum value. Accordingly, the secondcontrol grid 122 thereof serves only as an input electrode for the codegroup applied to this gate and the said grid may be operated at a fixedpotential which may be the potential of cathode 114 as shown.

In order to allow sufficient time for the complete transmission of eachof the code groups to the output terminal 63 of the selector, the signalapplied to amplifier 152 during the coding period, should not undergoamplitude variations greater than the increments between the biasvoltages supplied by the potentiometers 140. Preferably, during thecoding intervals, the control wave is maintained at constant amplitudevalues in the manner of a stepwave and such a wave may be derived fromthe signal to be coded by means of a quantizer of the type shown inFigure 4. The quantizer there shown comprises a storage capacitor 200adapted to be periodically charged and discharged by means of electrondischarge tubes 202 and 204.

The signal to be coded is applied to the quantizer through a suitableisolation amplifier 208 and a blocking condenser 210. It will be notedthat each of the tubes 202 and 204 is effectively in series with thestorage capacitor 200, the tube 202 having its cathode connected tocapacitor 200 and its anode connected to capacitor 210 whereas the tube204 is coupled with its anode to capacitor 200 and its cathode tocapacitor 210, so that the signal applied through capacitor 210 is theonly voltage appearing on the plates and cathodes of the tubes. Tubes202 and 204 are actuated at the repetition rate of the code groups, i.e., at a rate of 6 kc./sec. The actuating signal is preferably in theform of gating pulses of short duration which are applied to the tubesthrough a transformer 206 having individual output winding connected tothe control grid of the respective tubes.

When the signal applied to capacitor 210 has a positive going value, andwhen a gating pulse is applied to the grid of tube 202 through thetransformer 206, a charge in a given direction is produced on thecapacitor 200, the magnitude of the charge being determined by thecontemporaneous arrmlitude of the input signal. Negative 200`is...accordingly.modirzd proportionally toy-the changej .in theyamplitude ofwthe input .signal :occurring 'in the intere. 5 valsbetweenthel successive gating pu1`ses.`.,D.uring the` conductivesosthatsthesvoltage across.:.capaeitor-.200 remains 4constante@ By,so,peripdicallychargingE and dis- Chargingthe. capcitor.f200,..an inputsignal having a 'wave 10 :form such. as. shownA at:2011.rnay.-.be`convertedinto an l output-control signalsforathe .Selectorghavingastep-likQ are'produced;M 'These pulses, which are shown` at 232 andwhich,haveanepetition"raterequal to the "gatingfrequency lof theiquantizer(i.'-e:;'6-kc,/ sec'jjare appliedrtoithe trans- 1 formerMortsee Figure4)fto;`control atidsyn'chronze thel `operationrof'rthe'quantizerr `To?facilitate'theoperation ofthe'multivibrator230gitlie synchronizingsignal applied' thereto is'preferably lin"thefor'n'rjof shor'tduration pulses'produced,for'eXample"by means. of a differentiating net- *work 231.

similarlycausettube 204 :toconduct duringtheoccurrence j.

of the gating pulsestso. .that :thechargesonfthetcapacitor time` betweenpulsesneither: ofthetubeseZZ nor 204 is wave formsuchas shown at 203. tt, L .The code group. generator andetherquantizerare synn chronizedbyeatsynchroniaing signaltvgenerator,.one"suitf 154 able form .of whichis. shown/tin. Figureii;-Thefgenerator there shown. comprises/ta Waversourceft220-of..convcn `tional form which, fnfthemurpose ostabiIity,`lmay be .a piezo-electric crystal controlledfpscillator and. which, for

Y the-purpose; ,ofrsimplieityatoperatesl rataa frequency equal. 20lto.twice-'thepulsegrouprepetition rate,`.i...e.,.at 12.kc./ sec...

f The output oft-the sdurce 232,0:is-,appliedtto asuitable am.- -l`plitude-clipper.2225,diagrammaticallysillustratedfas a dual t diode.lclipper, ntenproduee w. a. square twave, signal. um The squaretwav'eysignaltso produced is supplied by rarst path.

l to a` diierentiatingiand .phase-ainvertingfnetwork 224 by t meansofwhichstwottrainsaof differentiated pulses recurring atvthe,rate vof1Q;tkciselaandiphasedalSOS apart, as

shown at 226 and Salvare-produced; Elhese .pulses are applied to thepulse group generator, i. e., to the sources 2 andw22 (see Figure.:2)..andffcontroland synchronize the operationthereofaspreviouslyfdescribed. The wave. at the output of clipper 222 issupplied by a secondpath^ nto"a multivibratorfrequencydivider 230'o conventional While theinvention has been described with speciiic reference to a four digitbinary coding system, it is readily apparent that the invention is alsoapplicable to binary coding systems utilizing a dilerent number ofdigits per code group. More particularly, by providing a code groupgenerator adapted to produce pulses at a greater rate per coding periodand by appropriately modifying the code group selector (i. e., byincreasing the number of selector gates to conform to the greaterdefinition so made available), a binary coding system based on the useof more than four digits per code group may be effected. Nor is theinvention restricted to coding systems in which the pulses representdigits of the binary number system. In this latter connection, it willbe noted that the system of the invention may be adapted to the use ofcode groups constructed in any desired manner so as to representdifferent amplitude values of the signal to be coded. ln such amodification of the invention there may be provided a series of codegroup generators, each continuously generating a dilerent code group ofpredetermined coniiguration at the desired coding rate which code groupsare supplied to the selector in the manner above described so as to beselectively supplied to the output of the system in a sequenceestablished by the amplitude Variations of the signal to be coded.

While I have described my invention by means of specific examples and ina specic embodiment, I do not wish to be limited thereto for obviousmodifications will occur to those skilled in the art without departingfrom the spirit and scope of the invention.

'WhatI-claim is:l r .1' Apparatus. forL transforming a signal'.lintoconsecutive .pulsecode groups representtive'of consecutivamplitudevaluesjof said signal,.compr.ising" 'means t"o simultaneously generateAin a.continuous manner 'a'pluality'of pu'lse code fgroups, eachl'of saidgroupsbeing'representative" of a different. -amplitude valuofsaid,signal, and means responi sive to.' said signal to consecutively selectsaid rcode' groups,

' each yof vlsaid selectedcode groups being representative of =theIcontemporaneous amplitude value`J of K,said signal. EA.2...Apparatus/.as claimedv in clai'rn lwherein said code groups. aregenerated. ata given rate' and'said signal responsive means selectssaidcode groups at said given rate. l ,3. Apparatusior ,transforming asignal into consecutive pulse..,.code.groups representative of.the"`ai'nplitude values of said signalat consecutive time-spaced'intervals, comprisingf means .to `sarrplesaid signal at" consecutiveintervals recurring, ata .given'frepetition rate tothereby produce.aquantizedfcontrol wavel having amplitude variations: inthe form ofdiscrete. steps, meansto simultaneously generate, in a .continuousmannerand at a'fzepetition rate equal to said given rate, a pluralityfof.pulse codel groups,

-each representtive'loa diierent step value1 of the amf. plitudeof.,said control wave; .and means responsive to said control Wavetoconsecutively select said. code groups, each of said selected codegroups beingrcpresentative of the contemporaneous amplitude valuef ofsaid control wave.

4. Apparatusasclaimed in claim 3 wherein said selecting means comprisesplurality of gates, each of said i gates "beingselectively, operativeytothe' exclusion of the 'others of saidgates as determined bytheamplitude of said control waveyja plurality-'of-'input paths,ea'chcoupled to a different one'iofsaidgatesgf rneans'tol supply'toeachof said'inputpaths'ardilterent oneiof sidfcode groups, and

acoinmonoutptpatli 'for said gates.

5 .""Appara'tus' `asclaimfe'd in claim 3' wherein said control wave hasagiven maximum number 'of discrete steps, wli'erei'rrth'el pulses of vthesaidJ groupsl occupy predetermined,tinta-spacedpositions-'andthe-maximum` number of combinations of'fthe pulses"'of'saidgroups'usequal to the. said `maximum `number of. discrete'steps'; wherein said 'jcode group generator'comprises individual circ'uirportionseachproduciii'ga 'differentoneiofsaid pulses'jand where- 'in` said"means`to supplyto each of "said pathsardierent one of said code groupscomprises means to couple each of said input paths to selected ones ofsaid circuit portions.

6. Apparatus for transforming a signal into consecutive pulse codegroups representative of the amplitude values of said signal atconsecutive time-spaced intervals, comprising means to sample saidsignal at consecutive intervals recurring at a given repetition rate tothereby produce a quantized control wave having amplitude variations inthe form of discrete steps, means to simultaneously generate in acontinuous manner and at a repetition rate equal to said given rate aplurality of pulse code groups, the pulses of which correspond to digitsof the binary number system, selector means having a plurality of inputchannels and a common output channel, means to apply a different one ofsaid pulse groups to a respective one of said input channels, and meansto apply said control wave to said selector, said selector meanscomprising means responsive to said control wave to couple a discreteone of said input channels to said output channel to the exclusion ofthe others of said input channels.

7. Apparatus as claimed in claim 6 wherein the said selector comprises aseries of gates arranged in cascade between a first terminal gate and alast terminal gate, each of the gates intermediate said terminal gatescomprising an electron discharge device having a cathode, an anode,first and second control electrodes arranged between said cathode andanode and an auxiliary electrode arranged intermediate said controlelectrodes, wherein the anodes of said gates are coupled in common tothe said output channel, wherein the said control wave is coupled to thesaid first control electrodes of said gates, wherein the said inputpaths are coupled to a respective one of said second control electrodes,and wherein the said auxiliary electrode of each of the said gates iscoupled to the second control electrode of the preceding gate of saidseries and applies a blocking signal to the said second electrode of thesaid preceding gate upon current ow between a cathode and auxiliaryelectrode of the succeeding gate.

8. Apparatus as claimed in claim 6 wherein the said pulse groupgenerator comprises a source adapted to generate a plurality of signalshaving a substantially square Wave form and phase displaced relative toeach other and further comprising means to convert said square wavesignals into pulses having a duration less than the period of saidsquare wave signals.

9. A code group generator comprising n sources of waves of the samefrequency but of different phase, where n is an integer greater than l,n means, each for deriving from an input wave a pulse coincident with agiven phase angle of said input wave, each of said means being suppliedfrom a diierent one of said sources, and a plurality of output circuitsrespectively supplied by the outputs of one or more of all said meansfor respectively combining said outputs to simultaneously obtain up towww! different combinations of pulses at said output circuits whilemaintaining Xed their phase displacement from one another, where pequals the number of pulses combined and may be any integer from one ton.

10. A generator as set forth in claim 9, wherein said sources providesquare waves and said rz means are responsive to a given maximum changeof slope of said square waves.

11. A code group generator comprising four sources of symmetrical squarewaves of the same frequency but in phase quadrature, means connected toeach source for deriving from each square wave a pulse of given polaritycoincident with a given change of direction of said wave, whereby theoutputs of said means comprise pulses of like polarities time phasespaced 90 apart, and a plurality of output circuits for combining theoutputs of said last means, one at a time; two at a time, three at atime, and four at a time, respectively whereby 15 possible combinationsof pulses are obtained.

12, A code group generator comprising a source of first synchronizingpulses, a source of second synchronizing pulses of the same repetitionfrequency as said rst pulses but out of phase therewith, a iirst doublystable multivibrator triggered by said source of first pulses forproducing two square wave outputs 180 out of phase with one another, asecond doubly stable multivibrator triggered by said source of secondpulses for producing two square wave outputs 180 out of phase with oneanother, four pulse generators, each generator being connected toreceive a separate square wave output of said multivibrators forderiving from the lagging edge of each square wave pulse, and aplurality of circuits for combining the outputs of said pulse generatorsone at a time, two at a time, three at a time, and four at a time,respectively.

13. A code group selector comprising a plurality of sources, each sourceproviding a different pulse code group of one or more pulses, a likeplurality of gate circuits, each of said circuits being connected to adifferent one of said sources, means providing a signal to be encoded,means coupled to said last named means and responsive to diiferentamplitudes of said signal for opening different ones of said gatecircuits, and means responsive to the opening of some of said gatecircuits for closing a previously opened gate circuit.

References Cited in the tile of this patent UNITED STATES PATENTS2,272,070 Reeves Feb. 3, 1942 2,409,229 Smith, Jr., et al. Oct. 15, 19462,437,707 Pierce Mar. 16, 1948 2,438,908 Goodall Apr. 6, 1948 2,529,666Sands Nov. 14, 1950 2,531,846 Goodall Nov. 28, 1950 2,541,039 Cole Feb.13, 1951 2,570,220 Earp et al. Oct. 9, 1951 2,579,302 Carbrey Dec. 18,1951 2,605,361 Cutler July 29, 1952 2,662,113 Schouten et al. Dec. 8,1953

